[Shiraz (Ray)]

Hi Alistair. Sounds like a good setup.

Because the refractive element will have dispersion, it's optical thickness will vary with wavelength and red will focus at a different position to blue after passing through. I am not an optics designer, but a back of the envelope calc showed that this longitudinal CA will be enough to produce enlarged spots at blue and red at f4 if imaging over the full 400-700 range. All is not lost though - if the camera has dropping sensitivity at both ends of the spectrum, it may not matter much. My camera has high blue sensitivity, so I was wary of getting an AO - I could not get any manufacturer's advice on how much CA to expect. If you are using filters though (RGB or narrowband), this is not an problem at all and there also should not be an issue at higher f numbers.

The bigger issue is that the AO will not be guiding on the same part of the sky as you are imaging in, so the seeing-induced motion that the guide cam is tracking will not be correlated with that in the image - eg if the seeing is moving your guide star left, the galaxy that you are imaging could well be moving to the right at the same instant, so the correction from the AO could easily make things worse. In the extreme, AO will make the stars nice and round, but 1.4x as big (from memory, I think that Rick found slight star enlargement with an AO). If you use the AO at lower frequency so that you are not influenced much by seeing, it should take out any residual mount errors, particularly those induced by wind. I don't think that AO can help get rid of most seeing induced tracking error with the systems we use, but it will certainly help with mount guiding errors at lower frequencies - be very interested in what you find. Regards Ray